Exploring Regioselective Bond Cleavage and Cross-Coupling Reactions using a Low-Valent Nickel Complex

Article abstract of DOI:10.1002/chem.201504959

Recently, esters have received much attention as transmetalation partners for cross-coupling reactions. Herein, we report a systematic study of the reactivity of a series of esters and thioesters with [{(dtbpe)Ni}₂(μ-η²:η²-C₆H₆)] (dtbpe=1,2-bis(di-tert-butyl)phosphinoethane), which is a source of (dtbpe)nickel(0). Trifluoromethylthioesters were found to form η²-carbonyl complexes. In contrast, acetylthioesters underwent rapid C_acyl-S bond cleavage followed by decarbonylation to generate methylnickel complexes. This decarbonylation could be pushed backwards by the addition of CO, allowing for regeneration of the thioester. Most of the thioester complexes were found to undergo stoichiometric cross-coupling with phenylboronic acid to yield sulfides. While ethyl trifluoroacetate was also found to form an η²-carbonyl complex, phenyl esters were found to predominantly undergo C_aryl-O bond cleavage to yield arylnickel complexes. These could also undergo transmetalation to yield biaryls. Attempts to render the reactions catalytic were hindered by ligand scrambling to yield nickel bis(acetate) complexes, the formation of which was supported by independent syntheses. Finally, 2-naphthyl acetate was also found to undergo clean C_aryl-O bond cleavage, and although stoichiometric cross-coupling with phenylboronic acid proceeded with good yield, catalytic turnover has so far proven elusive. The reactivity of a low-valent nickel complex with esters and thioesters is reported. Trifluoromethylthioesters were found to form η²-carbonyl complexes, while acetylthioesters were found to undergo further C_acyl-S oxidative addition and decarbonylation to generate methylnickel complexes. In contrast, the same nickel precursor was found to induce C_aryl-O bond cleavage in aryl esters to generate arylnickel acetate complexes (see scheme).

Full text of DOI:10.1002/chem.201504959

DOI: 10.1002/chem.201504959
Full Paper
&
Inorganic Chemistry
Exploring Regioselective Bond Cleavage and Cross-Coupling
Reactions using a Low-Valent Nickel Complex
Addison N. Desnoyer,[a] Florian W. Friese,[b] Weiling Chiu,[a] Marcus W. Drover,[a]
Brian O. Patrick,[a] and Jennifer A. Love*[a]
Abstract: Recently, esters have received much attention as
transmetalation partners for cross-coupling reactions. Herein,
we report a systematic study of the reactivity of a series of
esters and thioesters with [{(dtbpe)Ni}2(m-h2:h2-C6H6)]
(dtbpe=1,2-bis(di-tert-butyl)phosphinoethane), which is
a source of (dtbpe)nickel(0). Trifluoromethylthioesters were
found to form h2-carbonyl complexes. In contrast, acetyl-
thioesters underwent rapid CacylÀS bond cleavage followed
by decarbonylation to generate methylnickel complexes.
This decarbonylation could be pushed backwards by the ad-
dition of CO, allowing for regeneration of the thioester. Most
of the thioester complexes were found to undergo stoichio-
metric cross-coupling with phenylboronic acid to yield
sulfides. While ethyl trifluoroacetate was also found to form
an h2-carbonyl complex, phenyl esters were found to predo-
minantly undergo CarylÀO bond cleavage to yield arylnickel
complexes. These could also undergo transmetalation to
yield biaryls. Attempts to render the reactions catalytic were
hindered by ligand scrambling to yield nickel bis(acetate)
complexes, the formation of which was supported by inde-
pendent syntheses. Finally, 2-naphthyl acetate was also
found to undergo clean CarylÀO bond cleavage, and although
stoichiometric cross-coupling with phenylboronic acid
proceeded with good yield, catalytic turnover has so far
proven elusive.
Introduction
numerous transformations long considered to be particularly
challenging.[4]
In the development of new synthetic pathways, modern
chemistry strives towards the activation and functionalization
of kinetically inert bonds. Recently, much work has been done
in the area of CÀH and CÀO bond activation, allowing for the
development of new synthetic protocols that are both shorter
and more atom-economical than traditional approaches.[1]
While the catalytic reactivity of palladium and platinum to-
wards otherwise unreactive CÀX bonds (X=heteroatom) have
been well-documented for decades,[2] there is growing concern
about the cost and low natural abundance of these noble
metals for their continued use in industrial applications. Our
group is one of many currently interested in exploring the fun-
damental reactivity of nickel.[3] The unique redox reactivity of
nickel relative to its heavier congeners enables single-electron
transfer (SET) processes, which could result in reactivity com-
plementary to that of palladium and platinum. These charac-
teristics and the growing understanding of them have enabled
Among carbonyl compounds, esters show relatively low re-
activity towards nickel complexes, although there are several
early reports of CÀO bond cleavage using nickel(0).[5] Since
esters are a cheap and naturally abundant feedstock, the direct
activation of CÀO bonds of esters is of great interest.[6] The util-
ity of esters as the electrophilic source in Suzuki-type cross-
coupling reactions has recently been realized by Garg[7] and
Shi[8] using aryl pivalates with aryl boronic acids and aryl bor-
oxines, respectively. Subsequently, Itami succeeded in the isola-
tion and structural elucidation of an arylnickel(II) pivalate as an
intermediate in catalytic CÀH/CÀO biaryl coupling.[9] Our group
has also recently demonstrated decarbonylative cross-coupling
of esters with arylboronic acids by nickel catalysis as an
alternative to aryl halide-based Suzuki coupling.[10]
Interestingly, thioesters often show different activation path-
ways than esters. Jones’ recent report of platinum(0)-mediated
CÀS bond activation of aliphatic thioesters displayed an initial
CacylÀS bond cleavage yielding
a platinum acyl-thiolate
complex.[11] Upon heating, CO is released and activation of
a second equivalent of thioester led to a platinum dithiolate
complex. In addition, Riordan has reported that the use of
a comparable nickel(0) complex led to the cleavage of the
CacylÀS bond of thioesters containing an electron-deficient C6F5
substituent.[12]
[a] A. N. Desnoyer, W. Chiu, M. W. Drover, Dr. B. O. Patrick, Prof. Dr. J. A. Love
Department of Chemistry, The University of British Columbia
2036 Main Mall, Vancouver V6T 1Z1 (Canada)
E-mail: jenlove@chem.ubc.ca
[b] F. W. Friese
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster
Corrensstrabe 40, 48149 Münster (Germany)
We recently reported the synthesis of 2-nickelaoxetanes,[13]
prepared from the oxidative addition of [{(dtbpe)Ni}2(m-h2:h2-
C6H6)][14] (1) (dtbpe=1,2-bis(di-tert-butyl)phosphinoethane)
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201504959.
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onto epoxides that bear a ketone group. Our mechanistic stud-
ies indicated that the precoordination of the metal centre to
the carbonyl of the epoxide was crucial for the reaction to
occur. We thus sought to explore what other transformations
could be enabled by precoordination of a reactive, low-valent
nickel centre to a carbonyl fragment of organic molecules.
More broadly, we were also interested in exploring the funda-
mental organometallic reactivity of low-valent nickel com-
plexes with esters and thioesters to gain mechanistic insight
that could be useful for catalysis. Inspired by Sanford’s recent
work on decarbonylation of anhydrides,[15] we envisioned
a new route to the synthesis of RXCF3 (X=O, S) via the decar-
bonylation of esters and thioesters, respectively. A potential
mechanism for this transformation is shown in Scheme 1.
to coupling to both 31P and 19F nuclei. The solid-state structure
is shown in Figure 1, and demonstrates a CÀO bond length of
1.345(2) Š, which is elongated relative to most ketones.
We found that heating 3 at 708C in C6D6 in a sealed tube for
three days results in no observable reaction by 31P, 19F, and
1H NMR spectroscopic experiments. Even subsequent heating
at 110 8C for a further 24 h, results in no change to the NMR
spectra of 3. Thus, we conclude that the pendant CF3 group is
inactive towards thermally induced CÀF activation. Concomi-
tant with our studies on this system, Ogoshi and co-workers
published an elegant report on the CÀF activation of related
complexes using Lewis acids.[19]
Scheme 1. Potential synthesis of CF3XR.
A side reaction in the outlined catalytic cycle would be CÀF
activation of the pendant CF3 moiety, which is well known for
low-valent nickel complexes.[16] Indeed, Pçrschke has demon-
strated that 1 can activate C6F6 under thermolytic conditions.[14]
Thus, we sought to first prepare a trifluoromethyl ketone
complex derived from 1 and examine its reactivity.
Figure 1. POV-Ray (50% probability ellipsoids) diagram of 3.
We then turned our attention to other carbonyl-bearing sub-
strates (see Scheme 3). Addition of CF3-containing thioesters 4
and 5 to complex 1 results in the formation of h2-bound thio-
ester complexes 6 and 7. After workup, complexes 6 and 7 can
be isolated in 71 and 70% yield, respectively.
Results and Discussion
Addition of ketone 2 to a solution of 1 in benzene results in
an immediate color change from red/orange to yellow. Recrys-
tallization of the crude product from Et2O yields X-ray quality
crystals of the h2-ketone complex 3 (see Scheme 2) in 44%
isolated yield, which was characterized by multinuclear NMR
spectroscopic experiments, electron-impact mass spectrometry
(EI-MS), elemental analysis (EA), and X-ray diffraction studies.[17]
Scheme 3. Synthesis of complexes 6 and 7. Isolated yields in parentheses.
These complexes were fully characterized,[17] and the solid-
state structure of complex 6 is shown in Figure 2. Analogous
to complex 3, complex 6 shows a significantly elongated CÀO
bond length of 1.347(1) Š. The 31P NMR spectrum of complex 6
(see Figure 3) displays two resonances for the dtbpe ligand; an
Scheme 2. Synthesis of complex 3. Isolated yield in parentheses.
2
The NMR spectroscopic features of 3 are comparable to re-
lated complexes.[13,18] The coordination of the ketone to nickel
generates a C1-symmetric complex, with each of the four tBu
groups rendered inequivalent. The most notable feature is the
downfield-shifted resonance (d=74.5 ppm) of the carbonyl
carbon atom, which is broadened significantly, presumably due
[AB] doublet (d=93.1 ppm, JP,P =48 Hz) that we assigned as
the resonance for P2 (i.e. the phosphorous atom trans to the
oxygen atom of the thioester moiety) and a doublet of quar-
2
4
tets (d=90.1 ppm, JP,P =48, JP,F =11 Hz) that was assigned as
the resonance for P1. Dark-red, X-ray quality crystals of com-
plex 7 can be grown by the slow evaporation of a concentrated
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Figure 2. POV-Ray (50% probability ellipsoids) diagram of complex 6.
Figure 4. POV-Ray (50% probability ellipsoids) diagram of complex 7.
Scheme 4. Synthesis of complexes 11 and 12. Isolated yields from
metathesis reactions in parentheses.
Figure 3. 31P{1H} NMR spectrum (162 MHz, C6D6, 258C) of 6.
solution in toluene, and the solid-state structure of 7 is shown
in Figure 4.
In sharp contrast to the formation of h2-carbonyl complexes,
reacting acetylthioesters 8 or 9 with 1 yields a mixture of
products as determined by 31P NMR spectroscopy. One
product, present in both reactions, is readily identified as
[(dtbpe)Ni(CO)2] (10)[20] by its characteristic 31P NMR chemical
shift (d=94.8 ppm). The other main products in each case dis-
play two doublets with small coupling constants consistent
with oxidation to nickel(II).[21] Following purification by recrys-
tallization, complexes 11 (formed in 29% yield by 1H NMR
Figure 5. POV-Ray diagram of complex 11. Thermal ellipsoids shown at 50%
probability.
1
spectroscopy) and 12 (formed in 49% yield by H NMR spec-
the solid-state at room temperature over the course of two
weeks and in solution over a few days.
troscopy) were identified as methylnickel complexes derived
from decarbonylation of the thioester moiety. Complexes 11
and 12 could be prepared independently by a metathesis reac-
tion of a sodium thiolate with [(dtbpe)Ni(Me)I] (13) in 58 and
67% isolated yield, respectively (see Scheme 4).
In contrast to complex 11, complex 12 was found to be
thermally robust; heating solutions of complex 12 in C6D6 at
508C for one week resulted in no change to the 1H and
31P NMR spectra. Red/orange crystals of complex 12 could be
grown by cooling a saturated Et2O solution to À308C, and the
solid-state structure of complex 12 is shown in Figure 6.
Intrigued by the rapid decarbonylation of acetylthioesters,
we sought to explore whether the reaction could be pushed in
the reverse direction. Indeed, exposure of a solution of 12 to
The solid-state structure of complex 11 was determined by
single-crystal X-ray diffraction studies (see Figure 5). Although
analytically pure, dark-red crystals of complex 11 could be
grown by recrystallization from pentanes at À308C, complex
11 was found to be thermally sensitive, as it decomposes in
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Table 1. Stoichiometric cross-coupling of thioesters with PhB(OH)2.
R
R’
Yield [%][a]
CF3
CF3
CH3
CH3
Et
Ph
Et
35
73
0
Ph
12
[a] Yields determined by GC-FID analysis.
Figure 6. POV-Ray diagram of complex 12. Thermal ellipsoids shown at 50%
probability.
Scheme 6. Synthesis of complexes 15 and 16. [a]=Isolated yield.
[b]=1H NMR yield.
1 atm. of CO gas results in the formation of 10 as the major
organometallic species. In addition, thioester 9 is also formed,
1
as determined by GCMS and H NMR spectroscopy (84% yield,
see Scheme 5). Although at this point CO insertion into the
nickel–thiolate bond cannot be excluded, we propose that 9 is
formed by insertion of CO into the nickel–methyl bond of
12,[22 ]followed by reductive elimination.
Scheme 5. Reaction of 12 with CO to yield 9 and 10. [a]=31P NMR yield.
[b]=1H NMR yield.
Figure 7. POV-Ray diagram of complex 15. Thermal ellipsoids shown at 50%
probability.
While heating samples of 6 and 7 results in decarbonylation
leading to the formation of 10, we were unable to detect any
formation of RSCF3 (R=Et, Ph) by 1H and 19F NMR spectroscopy
or GCMS analysis. Given the success our group and others
have had using boronic acids as transmetalation reagents,[10,23]
we sought to explore the feasibility of CÀS bond formation by
combining complex 1, thioesters 4, 5, 8, or 9, and PhB(OH)2 in
one pot (see Table 1). Although we are able to form substitut-
ed sulfides in appreciable yields, we found that attempts to
render the cross-coupling reactions catalytic are hampered by
the formation of complex 10, which is unreactive to further
productive chemistry under the conditions employed.
see Scheme 6). Complex 15 was fully characterized,[17]and the
solid-state structure is shown in Figure 7.
Heating complex 15 in C6D6 at 608C results not in the for-
mation of EtSSEt, but rather the thioacetaldehyde complex 16
1
in 54% yield by H NMR spectroscopy. Concomitant formation
1
of EtSH is also observed in 46% yield by H NMR spectroscopy.
Eliminations of this type from zirconium dithiolates has been
reported previously by Buchwald.[26] Based on this, it seems un-
likely that the disulfides observed during the cross-coupling
are derived from 15. We currently hypothesize that the
presence of the disulfides is indicative of a radical process, and
further mechanistic studies are currently underway in our
laboratory.
Performing GCMS analyses confirmed the presence of disul-
fides (RSSR, R=Et or Ph) during the cross-coupling reactions.
Given that ligand-exchange reactions have previously been re-
ported for nickel(II) complexes,[24] we wondered if the disulfide
could be formed by reductive elimination from [(dtbpe)-
Ni(SR)2].[25] To probe this possibility, we prepared dithiolate 15
by reaction of two equivalents of NaSEt with [(dtbpe)NiCl2] (14,
We then sought to examine the corresponding chemistry of
1 with esters. While 1 was found to be unreactive with
10 equivalents of EtOAc, addition of trifluoromethyl derivative
17 to an orange/red solution of 1 results in a gradual color
change to yellow, and the h2-carbonyl complex 18 could be
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1
Scheme 9. Synthesis of complexes 22 and 23. H NMR yields in parentheses.
Scheme 7. Synthesis of complex 18. Isolated yield in parentheses.
bond cleavage followed by decarbonylation, analogous to the
thioester reactivity already discussed (vide supra). Complex 22
could also be prepared independently from a salt metathesis
reaction of NaOPh with 13.[17] The major product, complex 23
isolated in 59% yield after recrystallization of the crude prod-
uct from pentanes (see Scheme 7).[17] Crystals of 18 suitable for
X-ray diffraction analysis could be grown from slow evapora-
tion of a saturated toluene solution. The solid-state structure
of 18 (see Figure 8) is analogous to that of 6, including the
notably elongated CÀO bond length of 1.317(3) Š.
1
(58% yield by H NMR spectroscopy) was characterized as the
product of CarylÀO oxidative addition. Notably, the Houk group
has recently published
a comprehensive computational
work[27d] examining the origins of selectivity in Itami’s cross-
coupling of benzoxazole with aryl pivalates by using [(dcype)-
Ni(cod)] (dcype=1,2-bis(dicyclohexyl)phosphinoethane, cod=
1,4-cyclooctadiene) as a catalyst.[9] Based on their DFT calcula-
tions, Houk et al. propose that although CacylÀO bond cleavage
is energetically more accessible than CarylÀO bond cleavage,
subsequent decarbonylation would generate a sterically very
crowded tert-butylnickel intermediate, which prohibits further
reactivity.[27d] It is interesting to note here that although the
product of decarbonylation is much less sterically hindered,
the main product observed is still the result of CarylÀO oxidative
addition.
Although similar solubilities of 22 and 23 rendered purifica-
tion by recrystallization difficult, X-ray quality crystals of 23
could be grown by cooling a concentrated pentanes solution,
and the structure of 23 was confirmed by an X-ray diffraction
study (see Figure 9). To date, we have been unable to prepare
23 in pure form, either by purification of the reaction of 1 with
21 or by independent synthesis.
Figure 8. POV-Ray diagram of complex 18. Thermal ellipsoids shown at 50%
probability.
In contrast, 1 was found to react rapidly with phenyl ester
19 to give 20 in 59% isolated yield, which is the product of
CarylÀO bond cleavage (see Scheme 8).[27] Although we were
able to grow red crystals of complex 20, they were too disor-
dered to extract meaningful crystallographic data. Neverthe-
less, the structure of 20 can be assigned using NMR spectros-
copy.[17] In particular, the large 31P coupling constants for the
ipso-carbon atom resonance (d=150.8 ppm, dd, 2JC,P =80 Hz,
41 Hz) are indicative of a phenyl group bound to nickel.[28]
Figure 9. POV-Ray diagram of complex 23. Thermal ellipsoids shown at 50%
probability.
Scheme 8. Synthesis of complex 20. Isolated yield in parentheses.
To our surprise, reacting phenyl acetate 21 with complex
1 results in a complex mixture of products, as determined by
31P and 1H NMR spectroscopy[17] (see Scheme 9). One minor
product, formed in 18% yield by 1H NMR spectroscopy, was
characterized as complex 22, which is the product of CacylÀO
The dependence of which CÀX bond (X=O, S) is cleaved
during oxidative addition is outlined in Scheme 10. This type
of selectivity change is normally observed when altering the
phosphine ligand,[27d] and is unusual for a given chelated
nickel(0) system.
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Table 2. Cross-coupling of esters with boronic acids.
Scheme 10. Regioselectivity of bond cleavage of esters versus thioesters
with complex 1. [Ni]=[(dtbpe)Ni].
R
Ar
Yield [%] ArÀPh
Yield [%] 24
CH3
CH3
CH3
CF3
CF3
Ph
77
16
6
10
31
–
6
6
–
6
p-OMeC6H4
p-OCF3C6H4
Ph
Esters have recently received much attention as cross-cou-
pling partners in organic synthesis.[29] Thus, we envisioned that
20 and 23 could serve as intermediates in the cross-coupling
of aryl esters with boronic acids (see Scheme 11). While heating
20 at 708C for 24 h and reacting 20 with phenylboronic acid
at room temperature yielded only trace amounts of biphenyl,
refluxing a toluene solution of 1, esters 19 or 21, and PhB(OH)2
in stoichiometric amounts for 24 h results in the formation of
biphenyl 24 in 10 and 78% yield, respectively, as shown by
GC-FID analysis of the crude reaction mixture. Of note,
replacing PhB(OH)2 with Ph2Zn under identical reaction con-
ditions results in generally poorer yields of 24 (16 and 26%,
respectively).
p-OMeC6H4
[a] Yields determined by GC-FID analysis.
mixture that were suitable for an X-ray diffraction study (see
Figure 10).
These acetate complexes could be also be prepared inde-
pendently by the protonolysis of dimethyl complex 27 with tri-
fluoroacetic acid or acetic acid, as shown in Scheme 12. X-ray
quality crystals of 26 could be grown by cooling saturated
toluene solutions to À308C. The solid-state structure of 26 is
shown in Figure 11.
Scheme 11. Synthesis of biphenyl 24.
It is possible that the biphenyl observed could be formed by
the homocoupling of the boronic acid. Sanford has recently
reported on the homocoupling of phenylzinc reagents to
generate biphenyl.[15] We thus sought to explore the use of
alternate boronic acids to determine whether cross-coupling
or homocoupling was occurring in our system.
Notably, the yields of biaryl decrease dramatically when
using substituted arylboronic acids. Based on our results,
shown in Table 2, it seems clear that although both cross-cou-
pling and homocoupling are occurring in our system, the
cross-coupling process is the most prevalent. The low yields of
our system prompted us to study whether the biaryl product
was inhibiting turnover. Indeed, heating complex 1 with one
equivalent of biphenyl in C6D6 under our reaction conditions
led to the formation of a new complex as observed by
31P NMR spectroscopy, which we tentatively assign as [(dtbpe)-
Ni(biphenyl)]. To rule out any loss in yield due to undetected
Ni-bound biphenyl, C6F6 was added to the reaction upon
completion prior to GC-FID analysis.[17] However, the yields
remained unchanged.
Figure 10. POV-Ray diagram of complex 25. Thermal ellipsoids shown at
50% probability.
Scheme 12. Independent syntheses of complexes 25 and 26. Isolated yields
in parentheses.
Attempts to induce catalytic turnover were not met with
success, as lowering the loading of complex 1 results in a dra-
matic decrease in the yield of biaryl product. Indeed, during
the course of our cross-coupling studies, we noted the forma-
tion of a red precipitate as the reactions progressed. From
cross-coupling reactions of ester 19, red crystals of the bis(tri-
fluoroacetate) complex 25 could be isolated from the reaction
Naphthyl esters and ethers have been demonstrated to be
more reactive than their phenyl counterparts for cross-cou-
pling reactions with nickel[30] since their extended p-systems
are more capable of retaining aromaticity and stabilizing po-
tential Meisenheimer-type intermediates.[31] In an attempt to
improve our own cross-coupling, we prepared naphthyl ester
28 and reacted it with 1 (see Scheme 13).
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Unfortunately, while combining 1, 28, and PhB(OH)2 in THF
at 508C for 24 h results in the formation of 2-phenylnaphtha-
lene in 89% yield, as shown by GC-FID analysis, we were
unable to achieve catalytic turnover, as once again decreasing
the loading of nickel drastically decreased the yield of biaryl.
We are currently attempting to render the cross-coupling reac-
tions reported herein catalytic, as well as exploring further
mechanistic studies on these transformations.
Conclusion
We have examined the reactivity of a low-valent nickel com-
plex with a variety of esters and thioesters. While trifluorome-
thylthioesters were found to bind to nickel to form h2-carbonyl
adducts, acetylthioesters were found to undergo CacylÀS bond
cleavage, and subsequent decarbonylation to generate methyl-
nickel thiolate complexes. Most of the thioesters could under-
go cross-coupling with PhB(OH)2 to yield sulfides. In contrast,
aryl esters were found to typically undergo CarylÀO bond cleav-
age to yield aryl acetate complexes, which could also undergo
cross-coupling with arylboronic acids to yield biaryls.
Figure 11. POV-Ray diagram of complex 26. Thermal ellipsoids shown at
50% probability.
Scheme 13. Synthesis of complex 29. Nap=naphthyl. Isolated yield in
parentheses.
Experimental Section
General
See the Supporting Information for full experimental details.
Gratifyingly, the reaction produced
a single product,
CCDC 1440224 (3),
1440229 (12),
1440223 (6),
1440227 (15),
1440231 (7),
1440232 (18),
1440228 (11),
1440225 (23),
complex 29, which was recrystallized from pentanes in 40%
isolated yield and characterized as the naphthyl analogue of
23. The solid-state structure of 29 is shown in Figure 12.
1440226 (25), 440233 (26) and 1440230 (29) contain the supple-
mentary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre.
1
Interestingly, the H NMR spectrum shows two sets of tBu
peaks at room temperature: a sharp doublet at d=1.38 ppm
(3JH,P =11.9 Hz) and a broad multiplet at d=1.15 ppm, presum-
ably due to hindered rotation of the bulky naphthyl group.
Indeed, performing variable-temperature 1H NMR studies at
Acknowledgements
678C resolved the multiplet into
1.18 ppm, JH,P =12.8 Hz).[17]
a sharp doublet (d=
We thank the University of British Columbia, NSERC (Discovery
grant, Research Tools and Instrumentation grants) and the
Canada Foundation for Innovation for supporting this research.
A.N.D. is grateful to NSERC for a CGS-D and to the Izaak
Walton Killam foundation for a doctoral scholarship. W.C. is
grateful for support from the CREATE SusSyn program. M.W.D.
thanks the Government of Canada for a Vanier scholarship.
Keywords: cross-coupling · elimination · nickel · P ligands ·
sulfur
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Figure 12. POV-Ray diagram of complex 29. Thermal ellipsoids shown at
50% probability.
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Received: December 10, 2015
Published online on February 16, 2016
Chem. Eur. J. 2016, 22, 4070 – 4077
www.chemeurj.org
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